Crankcase volume effects on power

WWR

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#1
I know this question has been asked before, but I cant seem to find the thread.

Some performance shops claim great advantages to "stuffing" the crank with epoxy to reduce crankcase volume. How does this usually effect the power? Does it just effect throttle response, low end, or top end in any way?

This question is being asked in my unending attempt to build the "ultimate NSR engine". Since I am installing a new crank in my project engine, I will be able to do the full treatment to the cases. I have gasket-matched the mating surfaces where the cylinders meet, and rounded the sharp corners that follow the air-flow path to the cylinders. Would adding a little epoxy be worth it? What is the best type to use? What about durability issues?

Also, the jugs are currently being bored 2mm over by Eric. This in itself will create a greater primary compression ratio. Is there anything in particular to do to the bottom end that will correspond with this well for power enhancement?

The engine will be 266cc, 133cc per cylinder, and have a tuned peak of 11,200rpm. The pipes are made by Tyga Performance, tuned for the same peak.

I hope to attend Dirtweek '04, and this bike will be attending with me... :)
 

EricGorr

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#2
Crankcase volume and primary compression ratios are quite the conundrum, fills with opinion and theory. Look at the difference between a YZ125 and a KTM125. The YZ volume is relatively low and some tuners think that has advantages in transient throttle response. Whereas the KTM is a totally different story with a cavernous crankcase with a distintive sheilding around the flyweights of the crank, possibly to reduce the effects of windage or turbulence. So one might conclude that greater volume helps top end powerbands. If you do the obvious rounding of the sharp edges in the cases and smooth the transition from the reed box to the case transfers, I think that would be a good thing.

Perhaps Fastwes is reading this, he has a lot of experience with CR engines used in karts and dirt bikes, perhaps he's found some concrete rules on case volume for shifters.
 
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#3
You may not like my reply, it's not very scientific. I don't worry about crankcase volume! I've tried filling different parts of the cases and never saw any changes in dyno readings. It was downright disapointing! But most engines do respond to rounded or shaped edges like Eric said. Crankcase to transfer flow restrictions or variations make more differences. Obviously bike manufacturers get the whole different systems to work together, pipes, ignition,scavenging,compression(s) etc., even though they may be quite different from the other guys.
Wes Gilbert
 

WWR

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#4
Thanks guys!

I put some epoxy into the large divots where the oil passages connect to the curved air passage going up to the transfer ports (Honda put some really big pits right there). I retained the oil passage holes, but was able to dramatically smooth the airflow path toward the transfer ports. I pulled some pics of this from one of the major tuners' websites here, seems to be a common mod for the serious racers.